Hepatitis C virus (HCV) antibody was detected in 1499 donor sera by radioimmunoassay using an antigen expressed in yeast from a cDNA clone of the HCV genome. Eighteen samples over 4200 counts per minute (cpm) were considered to contain infectious HCV because these recipients developed typical posttransfusion non-A, non-B hepatitis after transfusion. The antibody-positive sera were all within the normal range of ALT levels. This assay system is thus useful for the screening for blood transfusion.